The Evolution of LINE-1 in Vertebrates

Area of Science:

• Genomics
• Evolutionary Biology
• Molecular Biology

Background:

• LINE-1 (L1) retrotransposons are mobile genetic elements with varying abundance and diversity across vertebrate species.
• Mammalian genomes are dominated by a few L1 families active sequentially, contrasting with the high diversity of concurrently active families in non-mammalian vertebrates.

Purpose of the Study:

• To investigate the evolutionary patterns of active L1 retrotransposons in mammals versus non-mammalian vertebrates.
• To understand the reasons behind the divergent abundance and diversity of L1 elements across vertebrate lineages.

Main Methods:

• Comparative genomic analysis of active L1 elements in 14 mammalian and 3 non-mammalian vertebrate species.
• Examination of L1 structure, base composition, codon bias, and domain evolution (5′UTR, ORF1, ORF2, 3′UTR).

Main Results:

• L1 elements exhibit substantial variation in length, base composition, and structure among vertebrates, with the 5′UTR and ORF1 showing the most divergence.
• Non-mammalian vertebrate L1s display species-specific features, indicating host-imposed functional constraints.
• Mammalian ORF1 evolves faster, and conserved 5′UTR and ORF1 in non-mammals suggest non-sequence-specific host repression.

Conclusions:

• Vertebrate L1 evolution is shaped by distinct host-pathogen dynamics, leading to different patterns of retrotransposon diversity and abundance.
• The absence of a sequence-specific arms race in non-mammalian vertebrates contrasts with the mammalian L1 evolutionary trajectory.